1. Overview
The safe and reliable operation of passenger vehicles, commercial trucks, military equipment, boats, aircraft, telecommunications equipment, electric vehicles, computer systems, and many other devices requires predictable and reliable performance of the batteries that are integrated into those systems. The present invention relates to battery monitoring equipment, more particularly to methods and apparatus for monitoring the condition of one or more batteries, and in particular for continuously monitoring performance characteristics of one or more batteries while the batteries are in use.
In the past it has been difficult to monitor the condition of the batteries used in such applications. One aspect of this difficulty relates to the many variables associated with battery systems, including dynamic and unpredictable duty cycles, loads, environments, connections, charging systems, battery age, battery to battery interactions, etc. In addition to the difficulty associated with monitoring battery systems subject to such variables, in many applications there is difficulty in conveniently installing a monitoring system. This difficulty relates to the remote location of the battery or batteries compared to the location of the operator. For example, in an automobile, the battery may be located in the engine compartment, while the operator, or the system that can benefit from battery performance information, is located in the cabin. This presents a wiring problem, especially if such a system is to be installed in a preexisting vehicle. Difficulties are also associated with distinguishing between a battery in good condition that happens to be discharged and a battery that has reached or is nearing the end of its useful lifespan. Finally, batteries are used in different circumstances that demand different monitoring techniques; more specifically, different techniques are appropriate for monitoring batteries used in engine starting service, which requires high currents drawn for short periods, than for batteries used in “deep-cycle” applications, wherein relatively lesser currents are typically drawn for longer periods.
More particularly, in many battery applications, for example, automotive and marine applications, it is useful and important for the vehicle system and the operator to understand the state of charge (“SOC”) of the battery, sometimes called the charge level, and the state of health (“SOH”) of the battery, sometimes called the battery life. SOC is often expressed as a percentage, such that a battery at 100% SOC is considered to be fully charged and a battery at 0% SOC is considered to be completely discharged. SOH is also commonly expressed as a percentage such that a battery exhibiting 100% SOH is considered to be new, and a battery at 0% SOH has reached the end of its useful life, being able only to store at full charge a fraction of the energy (typically set at 60%) it could store when new.
In particular, a class of automobiles commonly called stop-start vehicles automatically shut off the engine when the vehicle is slowing down or at rest at a traffic light, for example. When the vehicle is at rest with the engine turned off, all vehicle systems such as headlights, air conditioning, media equipment, etc. are being powered by the battery. When the driver wishes to accelerate, the vehicle automatically commands the engine to start so the vehicle can continue driving. The automatic engine control systems (ECS) in such vehicles require very accurate SOC information as the system must always ensure that there remains enough energy in the battery to start the engine. Based on SOC information, the ECS will allow the engine to be automatically turned off if it is determined that enough battery energy remains. Similarly, if SOC falls below a pre-determined threshold the ECS may command the engine to start in order to charge the battery even if the driver has not applied the accelerator.
SOH information is also important, primarily so as to allow the battery to proactively be replaced when it reaches the end of its useful life, rather than fail inconveniently while the vehicle is being used. More specifically, SOH measures the ability of a battery to store energy, which decreases as the battery ages. The SOH of a battery is derived by comparing the amount of energy the battery can store when fully charged compared to the amount of energy the battery could store when new. For example, a new battery of 100 ampere-hours (Ah) capacity may be considered to have reached end of life (SOH=0%) when it can only store 60 Ah of energy when fully charged.
Thus, for proper operation of a start-stop vehicle (and in many other battery applications), the SOH and SOC must be accurately evaluated to ensure that the battery always contains sufficient energy to function properly in the system.
2. Terminology
Different segments of the battery and related industries use the same terms, particularly “capacity”, differently; so that this application can be clearly understood, we define the following terms as indicated.
Present Capability (“Ca”)—A measure of the amount of energy stored by a battery at any given time.
Maximum capability (“Cm”)—A measure of the maximum amount of energy storable by a battery at any point over its entire lifespan.
Effective capability (“Ce”)—A measure of the maximum amount of energy storable by a battery at a given point in its lifespan.
End-of-life capability (“Ceol”)—A measure of the amount of energy storable by a battery at the end of its useful lifespan.
Thus, when a battery is in peak condition (typically shortly after entering service, as discussed further below) and is fully charged, its present capability Ca is equal to its effective capability Ce, and this is equal to its maximum capability Cm.
Over time, a battery's effective capability Ce decreases with respect to its maximum capability Cm; the relationship of Ce to Cm is termed the battery's state of health (“SOH”). When its effective capability Ce is equal to its end-of-life capability Ceol, it is considered to be beyond use. Ceol may be set arbitrarily, e.g., to 0.6 Cm. In order to express SOH as a percentage, such that SOH=100% when Ce=Cm and SOH=0% when Ce=Ceol, Ce−Ceol is divided by Cm−Ceol and multiplied by 100.
At any given time, a battery may be partially discharged, so that its present capability Ca is somewhat less than its effective capability Ce; the relationship of Ce to Ca is termed the battery's state of charge (“SOC”). As above, SOC can be expressed as a percentage by Ca/Ce×100.
3. Description of Related Art
The field of battery testing has been very active and many different technologies for evaluating various aspects of the condition of a battery have been developed. The principal approaches taken by the art are first categorized briefly below, for the purpose of describing the shortcomings of the various approaches, such that the benefits of the battery evaluation technologies used in the preferred embodiment of this invention can be readily appreciated.
Traditional methods for evaluating various different aspects of the condition of a battery include:                1. Establishing a pre-determined predicted voltage discharge curve for the battery, measuring the voltage between its terminals, and comparing the measured voltage to the pre-determined voltage discharge curve to determine its state of charge (SOC). This is referred to as “voltage sensing”.        2. Applying a heavy load to the battery, and measuring the voltage drop across its terminals. This method is referred to as “load testing”.        3. Establishing a reference point for the energy stored in a battery, measuring the current flowing into and out of the battery over time, typically by measuring the voltage drop across a shunt resistance or coil, and then estimating the total remaining energy compared to the reference point. This method is often referred to as “coulomb counting” or “current integration”, typically performed using a “VIT (voltage-current-temperature) sensor”.        4. Measuring the “dynamic conductance” of a battery by applying a time varying small amplitude AC signal to the battery for a period of time and measuring the voltage response, and then calculating the “dynamic conductance” of the battery based thereon; this value may be corrected with reference to the voltage across the battery, which is taken as indicative of its state of charge. See Champlin U.S. Pat. Nos. 3,909,708 and 4,912,416.        
The voltage sensing method mentioned first above has many shortcomings. First, the voltage curve of every battery model is unique and thus the reference curve must be specifically matched to a single battery model. Second, the actual discharge profile changes as the battery ages. Third, the discharge profile changes based on the size of the load. Lastly, the measured voltage will be significantly affected by the load (or charger) applied to the battery. For example, a fully-charged battery with a large load applied will display a low voltage, so that the battery will erroneously be deemed to be discharged.
The “load testing” method described second above, as typically practiced in the prior art, has the shortcoming, among others, of requiring significant current to be drawn from the battery, interfering with the accuracy of the measurement, and discharging and possibly damaging the battery.
The current shunt, “VIT”, or “coulomb counting” method mentioned third above also has many shortcomings. Most importantly, this method does not directly measure the level of energy in the battery, but only monitors the flow of energy into and out of the battery, and uses this data to measure departure from a prerecorded reference value. Over even a few discharge/recharge cycles the resulting estimation of capability drifts substantially and quickly from the actual energy in the battery due to inefficiencies associated with battery charging and discharging that cannot be measured or accurately estimated. Devices implementing this method are also large and cumbersome to install and connect.
More specifically, the “stop/start” vehicles discussed above require very accurate information concerning the SOC of their batteries. This has been attempted using VIT sensors. As above, VIT sensors measure current flow in and out of the battery, typically by measuring the voltage drop across a highly accurate shunt resistance while also measuring temperature. This data is sent to the vehicle computer or is analyzed by the sensor and SOC is evaluated by a process called current integration or coulomb counting. To achieve useful SOC results the sensor or vehicle computer must include a significant amount of detail about the battery and how it will be charged and discharged. This is necessary because the results of current integration must always be adjusted for factors that have large impacts on stored energy in the battery, but which cannot be measured by the current sensor. Such factors may include battery size, temperature sensitivity, rate capability, and other factors. Even with such detail, VIT sensors and the process of current integration over time are inherently prone to accumulated errors. While a VIT sensor may provide a highly accurate indication of SOC at the start of a measurement cycle, errors accumulate and can become substantial after 10 or 20 discharge cycles and often get worse as the battery ages.
In addition to being prone to accumulated error problems, because of the specific nature of the correction algorithms it is not possible to simply apply a VIT sensor to any battery in any application and expect to get useful SOC or SOH information. These shortcomings limit the application of VIT sensor technology to very specific applications for which the sensor has been programmed. These limitations are problematic for car makers and makers of other equipment that offer many models of equipment with many different batteries. Furthermore, the customers may use the products in many different ways and may replace the battery with respect to which the sensor was calibrated to function with different battery models available in the aftermarket. In each case the VIT technology is incapable of accurately measuring SOC.
The “AC signal” or “dynamic conductance” method mentioned lastly above also has significant shortcomings. The most significant shortcoming is that while the battery is in use, it will often be connected to loads or chargers producing noise or having components that respond to the applied AC signal. Thus, the measured voltage response will include significant distortion associated with devices attached to the battery. This deficiency makes it very difficult for this method to be used for accurately determining the condition of a battery while it is being used.
The prior art includes many patents directed to battery monitoring and evaluation. Exemplary approaches are disclosed in the following:
Tsuji U.S. Pat. No. 6,072,300 relates to characterization of the individual batteries of a large set of batteries. Internal resistance is estimated from cell voltage. See Col. 5, lines 32-38.
Fakruddin U.S. Pat. No. 5,027,294 also characterizes battery condition based on measurements of voltage.
Arai U.S. Pat. No. 6,201,373 shows a circuit for measuring the state of charge (SOC) of a battery, not a battery condition evaluation device per se. Voltage and current are both sampled.
Hirzel U.S. Pat. No. 5,381,096 also relates to SOC measurement.
Satake U.S. Pat. No. 6,531,875 teaches estimating the open circuit voltage of a battery based on extrapolation from a series of measurements.
Disser et al. US Patent Pub. No. 2003/0067221 A1 shows voltage regulator circuitry for automotive use.
Yokoo U.S. Pat. No. 5,828,218 shows a method for estimating residual capacity of a battery based on discharge current and voltage during discharge.
Munson U.S. Pat. No. 5,900,734 shows a battery monitoring system wherein the battery voltage is compared to a fixed reference value and an alarm is given when the battery voltage is less than the reference value.
Bramwell U.S. Pat. Nos. 5,721,688 and 6,097,193 discuss various methods of measuring the internal resistance and/or impedance of a battery, including application of a small AC signal to the battery and using a Wheatstone bridge or equivalent to measure the internal resistance. See col. 1, lines 40-48. Bramwell's claimed method includes the steps of measuring the impedance of a battery by sourcing to or sinking from the battery a current of known magnitude at intervals while the vehicle sits. Col. 9, lines 18-50.
Turner et al. U.S. Pat. No. 6,249,106 shows a circuit for preventing discharge of a battery beyond a predetermined point. Yorksie et al. U.S. Pat. No. 3,852,732 is directed toward the same objective. Finger et al. U.S. Pat. No. 4,193,026 is directed to measuring the SOC of a battery by integrating a signal indicative of reduction of the terminal voltage below a threshold value.
Reher et al. U.S. Pat. No. 5,130,699 shows a device for monitoring a battery by measuring the terminal voltage at regular intervals, comparing the measured values to a predetermined value, and setting a flag in a shift register depending on the result. When a predetermined number of flags indicate an under-voltage condition an alarm is given.
Sato et al. U.S. Pat. No. 5,193,067 discloses determining the internal impedance of a battery by measuring the voltage during discharge of a predetermined current, or by measuring the current during discharge at a predetermined voltage.
Slepian U.S. Pat. No. 5,764,469 shows disconnecting electronic equipment of a vehicle when the battery voltage falls below a predetermined level.
Gollomp et al. U.S. Pat. No. 6,424,157 refers to the difficulty of measuring battery SOC from open-circuit voltage (OCV) because this requires that the battery be disconnected. Gollomp instead teaches monitoring of the quiescent voltage (QV), e.g., measured at 30 minute intervals while the vehicle sits. Col. 9, lines 18-50. An alarm message can be given when QV falls below a predetermined point Col. 11, lines 28-39. Gollomp also teaches monitoring of voltage and current during engine starting (see FIG. 6). This data is stored in memory, see Col. 12, lines 48-50, and used to determine internal resistance (IR) and polarization resistance (PR). Gollomp also teaches monitoring SOC and QV over time to determine when the battery will not be able to start the car; see FIG. 3, Col. 14, line 22—Col. 16, line 36. Gollomp also teaches storing the first IR value of the battery, or some subsequent one, for “future use”—e.g., determination of IR change over time. PR is similarly monitored over time; see Col. 17, line 12- Col. 18, line 35. The result is to give warning of incipient battery failure or some problem with connections or the like. These data can be monitored during successive starts; see claim 1.
Kchao U.S. Pat. No. 5,751,217 shows a method and circuit for assessing battery impedance, which is stated to be applicable only to fully charged batteries, see Col. 3, lines 49-55, and Col. 4, line 12, and which is intended to be incorporated in a battery charger. By comparison, the apparatus of the invention is not limited to fully charged batteries and can be economically provided as a stand-alone unit or installed in a vehicle.
As noted above, it is also known to evaluate the condition of a battery by measurement of its “dynamic conductance”, that is, the inverse of its internal “dynamic resistance”, by applying a time-varying, small-amplitude AC signal to the battery for a period of time and measuring the voltage response, and then calculating the “dynamic conductance” of the battery based thereon; this value may be corrected with reference to the voltage across the battery, which is taken as indicative of its state of charge. See Champlin U.S. Pat. Nos. 3,909,708 and 4,912,416. However, this method is not suitable for measuring the dynamic conductance of a battery in use in the typically electronically “noisy” automotive environment.
Bertness U.S. Pat. No. 6,633,165 addresses measurement of the “cranking state of health” and “reserve state of health” of a battery by monitoring certain parameters of a battery, apparently preferably the dynamic conductance as measured according to the Champlin patents mentioned above.
U.S. Pat. No. 6,791,464 to Huang, one of the present inventors, incorporated herein by this reference, shows evaluation of the condition of a motor vehicle's battery by monitoring the voltage across the battery during starting, while the starter provides a substantial load. The minimum voltage reached during starting can be compared to a predetermined value to evaluate the condition of the battery.
U.S. Pat. No. 6,704,629 is also to Huang, and is also incorporated herein by this reference. According to the method disclosed in the Huang '629 patent, which can be considered a refinement of the known “load testing” technique described above, a relatively large known load is applied to a battery for a very short time. The voltage change and current flow associated with the very short transient load are measured. The DC internal resistance of the battery can be directly calculated from the voltage change and the current flow during the application of the known load. As the DC internal resistance is directly related to the remaining energy in the battery, this method directly measures battery capability. This method also eliminates the distorting effect of noise associated with connected equipment, and thus is considerably more useful than the AC signal method.
Commonly assigned U.S. Pat. No. 7,212,006 also to Huang, and also incorporated herein by this reference, relates to a method and apparatus for monitoring the condition of a battery by measuring its internal resistance (IR). The method involves measuring no-load voltages across the battery and a known load, connecting the load, measuring the loaded voltages, and determining IR based thereon. The method is capable of measuring the internal resistance of a battery while installed in an operational vehicle, that is, despite the presence of charge sources, such as an alternator, on the one hand, and loads on the other.
Commonly-assigned pending U.S. patent application Ser. No. 11/984,669 entitled “Method and Apparatus for Monitoring the Condition of a Battery By Measuring Its Internal Resistance”, incorporated herein by this reference, also to Huang, shows a further improvement in techniques for evaluating IR of a battery with respect to those disclosed in U.S. Pat. No. 7,212,006. This method involves connecting different known loads across the battery, measuring the load and battery voltages, and determining IR therefrom. Some of the methods and instruments disclosed herein requires measurement of IR of a battery during and after charging, and preferably employ the techniques disclosed in Ser. No. 11/984,669 for doing so, as discussed in detail below.